#a{
	width:1000px ;
	height: 1000px;
	background-color: aqua;
	margin: auto;
	line-height: 1000px;
	margin: 0;
}
#a #b {
	width: 500px;
	height: 500px;
	background-color: aliceblue;
	margin: auto;
	
	
}